The present invention relates generally to receivers, and more specifically to delay searchers for selecting candidate delays for the receiver.
Wireless signals often travel multiple propagation paths between a transmitter and an intended receiver. As such, the intended receiver receives a composite signal that typically includes multiple images of a transmitted signal, where each image generally experiences different path delay, phase, and attenuation effects. Different signal images therefore arrive at the receiver at different times, causing a delay spread between the received signal images. The maximum delay spread between signal images depends on, among other things, the differing characteristics of the signal propagation paths.
Because the signal energy is distributed among the multiple signal images, conventional wireless receivers often include one or more RAKE receivers, particularly in Code Division Multiple Access (CDMA) systems, such as IS-95, Wideband CDMA (WCDMA), and cdma2000 systems, to improve the signal-to-noise ratio (SNR) by combining the received signal images. RAKE receivers include a plurality of RAKE fingers tuned to different delays to despread signal images. Typically, the RAKE receiver tunes its available RAKE fingers to the strongest signal images, such that each selected signal image is despread and subsequently combined with the other selected and despread signal images. Combining multiple signal images in this manner generally improves the SNR of the received signal.
To support the despreading and combining operations, RAKE receivers include, or otherwise cooperate with, a path searcher that identifies one or more signal energies in a received signal across a defined search window. Conventional path searchers generate a signal energy vs. delay profile and search through the profile to identify candidate delays. In some systems, the path searcher may use peak detection to identify the candidate delays.
Other systems may overlay a grid onto the signal energy vs. delay profile and compare the signal energy at each grid point to a threshold. The grid points with signal energies that meet or exceed the threshold are selected as candidate delays. In either of these cases, the candidate delays are available to the RAKE receiver for assignment to the RAKE fingers.
Because channel parameters normally change over time, the path searcher may continue to monitor the received signal to track current RAKE finger delays and to search for new candidate delays. Further, because reassigning a RAKE finger necessarily requires that the reassigned RAKE finger be disabled for a period of time, the path searcher may also perform a verification function to prevent unnecessary reassignment of RAKE fingers. In general, the verification function evaluates the candidate delays over time to verify that a new candidate delay has been found, that a current RAKE finger delay is no longer present in the received signal, and/or that a current RAKE finger delay is still present in the received signal.
There are several approaches for implementing a verification function in a RAKE receiver. One conventional approach simply averages a signal energy level corresponding to a certain delay over multiple measurements, i.e., by using a linear average, an exponential average, or other forms of low pass filtering. If the averaged signal energy level meets or exceeds a threshold, the path searcher verifies the corresponding delay as a viable RAKE finger delay.
Other conventional methods compare multiple measurements of a signal energy level corresponding to a certain delay to a threshold, as shown in “A New Receiver for Asynchronous CDMA: STAR—the Spatio-Temporal Array Receiver,” published in IEEE J-SAC, vol. 16, no. 8, pp. 1411-1422, October 1998, or compare a fraction of consecutive signal energy level measurements at a certain delay to a threshold, as shown in “Performance Analysis of DS-SS PN Code Acquisition Systems Using Soft Decision Techniques in a Rayleigh-fading Channel,” published in IEEE T-VT, vol. 51, no. 6, pp. 1587-1595, November 2002, both of which are incorporated herein by reference. In any event, the object of the verification function is (1) to prevent unnecessary reassignment of the RAKE fingers, and (2) to enable the discovery of new paths.